ci: fix workflow - use docker:27-dind container with git built-in

Previous run failed because:
- rust:1.83-bookworm container lacks Node.js (needed for actions/checkout)
- Docker not available in the rust container
- actions/checkout@v4 is a third-party action that needs Node

Solution:
- Use docker:27-dind container (has docker + git preinstalled)
- Replace actions/checkout with direct git clone + checkout
- All steps run directly in the docker container

This matches Forgejo's native environment better than trying to run
GitHub Actions inside a Rust container.

.gitea/workflows/build.yaml

@@ -8,17 +8,22 @@ on:
 jobs:
   build-and-push:
     name: Build and Push Image
-    # Use 'rust' runner (not 'docker')
-    # Available runners in Forgejo: golang, rust, node
-    # rust runner provides: Rust 1.83-bookworm + Docker-in-Docker for image builds
     runs-on: rust
+    container:
+      image: docker:27-dind
+      options: --privileged
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        run: |
+          apk add --no-cache git
+          git clone https://git.riotpiao.com:2222/rock/poimen-memory.git /workspace
+          cd /workspace
+          git checkout ${{ github.sha }}
 
       - name: Get commit info
         id: info
         run: |
+          cd /workspace
           SHORT_SHA=$(git rev-parse --short HEAD)
           COMMIT_MSG=$(git log -1 --pretty=%B | head -1)
           echo "short_sha=${SHORT_SHA}" >> $GITHUB_OUTPUT
@@ -32,6 +37,7 @@ jobs:
 
       - name: Build image
         run: |
+          cd /workspace
           docker build \
             --tag forgejo.riotpiao.com/rock/poimen-memory:${{ steps.info.outputs.short_sha }} \
             --tag forgejo.riotpiao.com/rock/poimen-memory:latest \